Oil sand, as known in the Athabasca region of Alberta, Canada, comprises water-wet, coarse sand grains having flecks of a viscous hydrocarbon, known as bitumen, trapped between the sand grains. The water sheaths surrounding the sand grains contain very fine clay particles. Thus, a sample of oil sand, for example, might comprise 70% by weight sand, 14% fines, 5% water and 11% bitumen (all % values stated in this specification are to be understood to be % by weight).
For the past several decades, the bitumen in Athabasca oil sand has been commercially recovered using a water-based process. In the first step, the oil sand is slurried with process water, naturally entrained air and, optionally, caustic (NaOH). The slurry is mixed, for example in a tumbler or pipeline, for a prescribed retention time, to initiate a preliminary separation or dispersal of the bitumen and solids and to induce air bubbles to contact and aerate the bitumen. This step is referred to as “conditioning”.
The conditioned slurry is then further diluted with flood water and introduced into a large, open-topped, conical-bottomed, cylindrical vessel (termed a primary separation vessel or “PSV”). The diluted slurry is retained in the PSV under quiescent conditions for a prescribed retention period. During this period, aerated bitumen rises and forms a froth layer, which overflows the top lip of the vessel and is conveyed away in a launder. Sand grains sink and are concentrated in the conical bottom. They leave the bottom of the vessel as a wet tailings stream containing a small amount of bitumen. Middlings, a watery mixture containing fine solids and bitumen, extend between the froth and sand layers.
The wet tailings and middlings are separately withdrawn, combined and sent to a secondary flotation process. This secondary flotation process is commonly carried out in a deep cone vessel wherein air is sparged into the vessel to assist with flotation. This vessel is referred to as the Tailings Oil Recovery (TOR) vessel. The bitumen recovered by flotation in the TOR vessel is recycled to the PSV. The middlings from the deep cone vessel, termed as flotation tailings are sent to tailings pond. The underflow from the deep cone vessel, i.e., the coarse tailings, is pumped through pipeline to the tailings deposition areas. In the alternative, a series of flotation cells can be used to recover the bitumen remaining in the wet tailings and/or middlings from the PSV.
The bitumen froths produced by the PSV are subjected to cleaning, to reduce water and solids contents so that the bitumen can be further upgraded. A typical bitumen froth obtained from the PSV comprises about 60-65 wt % bitumen, about 25-30 wt % water and about 10 wt % solids. There are currently two different types of PSV bitumen froth treatment processes which are used in the oil sands industry. One type of froth treatment process is the naphthenic process, which has been used commercially for several decades. The other type of froth treatment process is the paraffinic process, which has been developed more recently. Both types of froth treatment use a solvent to produce a diluted bitumen product (i.e., dilbit) which is diluted with the solvent.
More particularly, with respect to the naphthenic process, bitumen froth is diluted with the light hydrocarbon diluent, naphtha, to increase the difference in specific gravity between the bitumen and water and to reduce the bitumen viscosity, to thereby aid in the separation of the water and solids from the bitumen. This diluent diluted bitumen froth is commonly referred to as “dilfroth”. It is desirable to “clean” dilfroth, as both the water and solids pose fouling and corrosion problems in upgrading refineries. By way of example, the composition of naphtha-diluted bitumen froth typically might have a naphtha/bitumen ratio of 0.65 and contain 20% water and 7% solids. It is desirable to reduce the water and solids content to below about 3% and about 1%, respectively, to make it amenable to further upgrading. Separation of the bitumen from water and solids may be done by treating the dilfroth in a sequence of scroll and disc centrifuges. Alternatively, the dilfroth may be subjected to gravity separation in a series of inclined plate separators (“IPS”) in conjunction with countercurrent solvent extraction using added light hydrocarbon diluent.
In the paraffinic process, a paraffinic solvent is used to dilute the bitumen contained in the bitumen froth. A paraffinic solvent consists of or contains significant amounts of one or more relatively short-chained aliphatic compounds (such as, for example, C4 to C8 aliphatic compounds). Asphaltenes generally exhibit less solubility in paraffinic solvents than in naphtha solvents, and asphaltenes tend to exhibit greater solubility in longer chain paraffinic solvents than in shorter chain paraffinic solvents.
In the paraffinic process, the addition of the paraffinic solvent to the bitumen froth appears to destabilize the asphaltenes contained in the bitumen froth, some of which precipitate out as clusters or aggregates while simultaneously trapping maltenes, solid mineral material and water within the clusters and aggregates. The precipitation of asphaltenes therefore has the effect of separating solid mineral material and water from the bitumen, while the increased difference in specific gravity between the phases which results from the dilution of the bitumen (including both maltenes and un-precipitated asphaltenes) by the paraffinic solvent enhances the separation of the remaining solid mineral material and water from the diluted bitumen. Typically, the paraffinic process is performed in a manner so that between about 40 percent and about 50 percent by weight of the asphaltenes contained in the bitumen froth are precipitated in order to produce a diluted bitumen product which has a relatively low solids and water content.
However, when bitumen froths are obtained from more non-traditional sources, e.g., from oil sand tailings, fluid fine tailings, middlings, and the like, the composition of these froths are not amenable to conventional froth treatment processes. For example, a typical bitumen froth obtained from fluid fine tailings using flotation based technologies comprises about 10-20% bitumen, about 60-70 wt % water and about 20% solids. Therefore, there is a need for a froth treatment process that can be used to extract fungible bitumen from low grade bitumen froth.